Because Windows Movie Maker would stretch it if rotated, otherwise.
Why won't you use any other movie maker? The windows one is just bad and there is many freeware.
Because the freeware ones are ridiculously hard to use and hard to find (since many shareware ones tricks you into thinking it's free, yet it's not). I downloaded one once that was really free and nothing in it seemed to do anything. Normally I would have used VirtualDub, but my iPod touch produces mov files, which VirtualDub cannot open. Also Movie maker lets me make a much faster job for most other tasks such as the Omni video tributes.